1. Osmoregulation and Excretion
Chapter 44
Osmoregulation and Excretion

2. Overview: A Balancing Act
Physiological systems of animals operate in a fluid environment.
Relative concentrations of water and solutes must be maintained within fairly narrow limits.
Osmoregulation regulates solute concentrations and balances the gain and loss of water.

3. Freshwater animals show adaptations that reduce water uptake and conserve solutes.
Desert and marine animals face desiccating environments that can quickly deplete body water.
Excretion gets rid of nitrogenous metabolites and other waste products.

4. Osmoregulation balances the uptake and loss of water and solutes
Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment. Cells require a balance between osmotic gain and loss of water.
Osmolarity = the solute concentration of a solution, determines the movement of water across a selectively permeable membrane.
If two solutions are isoosmotic, the movement of water is equal in both directions.
If two solutions differ in osmolarity, the net flow of water is from the hypoosmotic to the hyperosmotic solution.

5. Selectively permeable membrane
Solute concentration and osmosis
Selectively permeable
membrane
Solutes
Net water flow
Water
Figure 44.2
Hyperosmotic side
Hypoosmotic side

6. Osmotic Challenges
Osmoconformers, consisting only of some marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity.
Osmoregulators expend energy to control water uptake in a hypoosmotic environment and loss in a hyperosmotic environment.

7. Most animals are stenohaline; they cannot tolerate substantial changes in external osmolarity.
Euryhaline animals can survive large fluctuations in external osmolarity.

8. Marine Animals
Most marine invertebrates are osmoconformers.
Most marine vertebrates and some invertebrates are osmoregulators.
Marine bony fishes are hypoosmotic to sea water. They lose water by osmosis and gain salt by diffusion and from food.
They balance water loss by drinking seawater and excreting salts.

9. Osmoregulation in marine and freshwater bony fishes: a comparison: drinking, gills, urine …
Gain of water and
salt ions from food
Excretion
of salt ions
from gills
Osmotic water
loss through gills
and other parts
of body surface
Uptake of water and
some ions in food
Uptake
of salt ions
by gills
Osmotic water
gain through gills
and other parts
of body surface
Gain of water
and salt ions from
drinking seawater
Excretion of salt ions and
small amounts of water in
scanty urine from kidneys
Excretion of large
amounts of water in
dilute urine from kidneys
Figure 44.4
Osmoregulation in a saltwater fish
Osmoregulation in a freshwater fish

10. Freshwater Animals
Freshwater animals constantly take in water by osmosis from their hypoosmotic environment.
They lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine.
Salts lost by diffusion are replaced in foods and by uptake across the gills.

11. Animals That Live in Temporary Waters
Some aquatic invertebrates in temporary ponds lose almost all their body water and survive in a dormant state.
This adaptation is called anhydrobiosis.

12. Anhydrobiosis - adaptation… Hydrated = active state dehydrated = dormant state.
Figure 44.5
(a) Hydrated tardigrade
(b) Dehydrated
tardigrade

13. Land Animals
Land animals manage water budgets by drinking and eating moist foods and using metabolic water.
Desert animals get major water savings from simple anatomical features and behaviors such as a nocturnal life style.

14. Water balance in two terrestrial mammals
balance in a
kangaroo rat
(2 mL/day)
Water
balance in
a human
(2,500 mL/day)
Ingested
in food (0.2)
Ingested
in food (750)
Ingested
in liquid (1,500)
Water
gain
(mL)
Derived from
metabolism (1.8)
Derived from
metabolism (250)
Figure 44.6
Feces (0.09)
Feces (100)
Water
loss
(mL)
Urine
(0.45)
Urine
(1,500)
Evaporation (1.46)
Evaporation (900)

15. Energetics of Osmoregulation
Osmoregulators must expend energy to maintain osmotic gradients. Animals regulate the composition of body fluid that bathes their cells.
Transport epithelia are specialized epithelial cells that regulate solute movement.
They are essential components of osmotic regulation and metabolic waste disposal. They are arranged in complex tubular networks
An example is in salt glands of marine birds, which remove excess sodium chloride from the blood.

16. How do seabirds eliminate excess salt from their bodies?
EXPERIMENT
Nasal salt
gland
Ducts
Nostril
with salt
secretions
Figure 44.7

17. Countercurrent exchange in salt-excreting nasal glands
Vein
Artery
Secretory
tubule
Secretory
cell
Salt gland
Countercurrent exchange in salt-excreting nasal glands
Capillary
Secretory tubule
Transport
epithelium
NaCl
NaCl
Direction of
salt movement
Figure 44.8
Central duct
Blood
flow
Salt secretion
(a)
(b)

18. An animal’s nitrogenous wastes reflect its phylogeny and habitat
The type and quantity of an animal’s waste products may greatly affect its water balance.
Among the most important wastes are nitrogenous breakdown products of proteins and nucleic acids.
Some animals convert toxic ammonia (NH3) to less toxic compounds prior to excretion.

19. Nitrogenous wastes Proteins Nucleic acids Ammonia Urea - less toxic
Amino
acids
Nitrogenous
bases
Amino groups
Most aquatic
animals, including
most bony fishes
Mammals, most
amphibians, sharks,
some bony fishes
Many reptiles
(including birds),
insects, land snails
Figure 44.9
Ammonia
Very toxic
Urea - less toxic
Uric acid - not soluble

20. Animals Excrete Different Forms of Nitrogenous Wastes
Ammonia - needs lots of water. Animals release ammonia across whole body surface or through gills / aquatic animals.
Urea - The liver of mammals and most adult amphibians converts ammonia to less toxic urea. The circulatory system carries urea to kidneys, where it is excreted. Conversion of ammonia to urea is energetically expensive; uses less water than ammonia.

21. Nitrogenous Wastes …
Uric Acid - Insects, land snails, and many reptiles, including birds, mainly excrete uric acid. Uric acid is largely insoluble in water; can be secreted as a paste with little water loss. Uric acid is more energetically expensive to produce than urea.
The kinds of nitrogenous wastes excreted depend on an animal’s evolutionary history and habitat.
The amount of nitrogenous waste is coupled to the animal’s energy budget.

22. Diverse excretory systems are variations on a tubular theme
Excretory systems regulate solute movement between internal fluids and the external environment. Most excretory systems produce urine by refining a filtrate derived from body fluids.
Key functions of most excretory systems:
Filtration: pressure-filtering of body fluids
Reabsorption: reclaiming valuable solutes
Secretion: adding toxins and other solutes from the body fluids to the filtrate
Excretion: removing the filtrate from the system.

23. Key functions of excretory systems: an overview
Filtration
Blood --> tubule
Capillary
Filtrate
Excretory
tubule
Reabsorption
Tubule --> blood
Figure 44.10
Secretion
Urine
Excretion

24. Survey of Excretory Systems
Systems that perform basic excretory functions vary widely among animal groups. They usually involve a complex network of tubules.
Protonephridia flame cells / planaria
Metanephridia earthworm / similar to nephrons
Malpighian Tubules insects
Nephrons = the function unit of the kidneys / humans.

25. Protonephridia
A protonephridium is a network of dead-end tubules connected to external openings.
The smallest branches of the network are capped by a cellular unit called a flame bulb.
These tubules excrete a dilute fluid and function in osmoregulation.

26. Protonephridia: the flame bulb system of a planarian
Nucleus
of cap cell
Cilia
Flame
bulb
Interstitial
fluid flow
Tubule
Opening in
body wall
Figure 44.11
Tubules of
protonephridia
Tubule cell

27. Metanephridia
Each segment of an earthworm has a pair of open-ended metanephridia.
Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion.

28. Metanephridia of an earthworm
Coelom
Capillary network
Components of
a metanephridium:
Internal opening
Figure 44.12
Collecting tubule
Bladder
External opening

29. Malpighian Tubules
In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation.
Insects produce a relatively dry waste matter, an important adaptation to terrestrial life.